Polyethylene is the most widely used commercial polymer. It can be converted to finished articles by a couple of different processes. A major application for polyethylene is for example preparing films. An important factor which influences the quality of the polymer films is the number of visible defects in the films, the so-called gels. The more gels are present in the film the lower is its value. Above a certain boundary value such films can even be unmarketable. Gels and especially larger gels are however not only disadvantageous for preparing films but also for other applications of polyethylene because they can impair, for examples, surface quality or even mechanical properties of produced articles.
Gels in polyethylenes can come from several sources. They can be caused, for example, by residues from catalysts or additives or by contamination. They can be the result of crosslinked material, e.g. caused by overheating. Gels can however also be the result of non-homogenized high-molecular weight fractions, for example in bimodal or multimodal resins. There have been many approaches to reduce the number of gels in polymer films. One of them, especially for reducing the number of gels caused by non-homogenized high-molecular weight fractions, is filtering the polymer melt.
WO 98/24843 A1 teaches a method for reducing the number and size of polymeric gels in polypropylene blends by passing the polypropylene blend through a plurality of screen filters, each screen filter having openings of 44 μm to 900 μm.
EP 848 036 A1 discloses a process for preparing a polyethylene blend by preparing two polyethylenes, blending them, melting the blend and, prior to extrusion or pelletizing, passing the molten blend through one or more active screens of a retention size in the range of 2 nm to 70 μm. WO 2004/101674 A1 also discloses melt screening multimodal polyethylene compositions prepared in a cascade of fluidized bed reactors through one or more active screens having a retention size in the range of 2 μm to 70 μm.
WO 2007/053258 A1 teaches a process of producing a polyolefin by providing a polyolefin; forming a melt of the polyolefin and passing the polyolefin through one or more active screen filter(s) having a mesh size of from 70 μm to 200 μm at a mass flux of from 3,500 kg/hr/m2 to 70,000 kg/hr/m2; and isolating the polyolefin having passed through the screen filter.
All these methods for reducing the number of larger gels in polyethylene films require relatively fine screens which are mechanically less stable than coarser screen and which have a higher tendency for plugging than coarser screens.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to find a method for reducing the amount of larger gels in polyethylene compositions with mechanically more stable screens and a lower risk of plugging of the screens.